Biodiversity is important for providing ecosystem services, such as increasing plant growth and carbon storage. In forests, canopy complexity – the 3-dimensional shape and structure of tree canopies – is related to both the number of species present and how fast the forest grows, but it's hard to tell which factor is causing what.To better understand these relationships, we studied two large tree experiments—one at the Smithsonian Environmental Research Center (SERC) and another at the National Zoo’s Conservation Biology Institute (NZCBI). These experiments include forest plots planted with different numbers of tree species (1 species, 4 species, or 12 species).

We found that more diverse forest plots had 71% more complex canopies and 73% faster growth. Having more complex canopies was the main driver of faster tree growth in diverse forests. We also found differences between the two sites. At the more productive SERC site, we found a strong relationship between diversity and tree growth, but not at the slower growing NZCBI site which has overall less complex canopies.Overall, the study shows that having a more layered forest canopy is a key reason why having diverse tree species makes forests more productive.